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Old clams give up ocean climate secrets

Like rings in a tree trunk, layers in shells of quahog clams provide clues to the climate over the past 1,000 years. Amy Middleton reports.

Quahog clams can live up to 500 years – living archives of the climate of the period.

Paul Poplis / Getty Images

The world’s longest-living animal has revealed 1,000 years’ worth of ocean data and interactions with global climate, new research has found.

The quahog clam, an edible mollusc native to North America and Central America, lives for up to 500 years. Its age is depicted by the rings on its hard shell, much like the inside of a tree trunk.

As well as reflecting shell growth, the chemical makeup of these rings can hold clues to the state of the ocean over centuries – or in this case, a full millennium, thanks to the analysis of almost 1,500 shell samples by an international research team.

The quahog clams were collected on the North Icelandic shelf – a spot ideal for examining ocean dynamics because it reflects the interplay between two very distinct water masses: the warm and salty subpolar mode water, and the cool and fresh Arctic intermediate water.

The researchers analysed the ratio of different oxygen isotopes – which differ slightly in weight – in the shells and compared them to records from oceanographic instruments to shine some light on what drove significant climate shifts.

“Our results show that solar variability and volcanic eruptions play a significant role in driving variability in the oceans over the past 1,000 years,” says lead researcher David Reynolds at Cardiff University, UK.

“Results also showed that marine variability has played an active role in driving changes to northern hemisphere air temperatures in the pre-industrial era.”

But the researchers found this trend stopped during the industrial era, where temperature changes in the northern hemisphere were instead induced by human-induced greenhouse gas emissions.

Co-author of the study Ian Hall says for accurate climate modelling, climate scientists need to incorporate ocean activity before human-induced greenhouse gas began making an impact.

“Our results highlight the challenge of basing our understanding of the climate system on generally short observational records.